As a result of investigation for imparting a water resisting property to a support for photographic papers to provide for quick development of the photographic papers, a waterproof support for photographic papers composed of a paper coated with polyethylene on both surfaces thereof has been developed and widely used.
In a photographic paper prepared by coating on the surface of such a waterproof support silver halide emulsion layers, a severe curling (bending) phenomenon occurs with the silver halide emulsion layer at the inner side, and hence the fundamental solution to the foregoing problem as a problem peculiar to a waterproof paper has been desired. Therefore, the cause of the aforesaid curling phenomenon was investigated by analyzing the production steps of waterproof papers, and as the result thereof it was confirmed that when a photographic silver halide emulsion coated on the surface of a support is dried to form a silver halide emulsions layer in solid state, a shrinkage of volume occurs to cause a shrinking stress inside of the emulsion layer, whereby curling occurs with the emulsion layer at the inner side. If such curling occurs, the photographic paper thus finished cannot be properly maintained in a tabular state, and handling of the photographic paper becomes troublesome owing to the bending, thus greatly reducing the commercial value of the photographic paper.
As one method of solving such a curling problem, a method has been proposed for obtaining a photographic paper having no curling by previously curling the support with the surface of the side to be coated with silver halide emulsions at the outer side, and then coating the photographic silver halide emulsions thereon to balance the curling with the shrinking stress occuring during the drying step of the photographic paper. For example, in a polyethylene-coated paper prepared by casting polyethylene in a molten state onto the surfaces of a travelling paper base, the following methods are industrially practiced. First, there is a method of providing curling to the support with the surface of the side to be coated with silver halide emulsions on the outer side by making the thickness of the polyethylene layer at the back side of the support thicker than the thickness of the polyethylene layer on the side to be coated with silver halide emulsions by utilizing the shrinking property of polyethylene, as shown in U.K. Pat. No. 1,269,802. Second, there is a method of providing curling to the support with the surface of the side to be coated with silver halide emulsions on the outer side by making the density of the polyethylene layer on the back side of the support higher than the density of the polyethylene layer on the side to be coated with silver halide emulsions by utilizing the shrinking property caused by the density of polyethylene, as shown in Japanese Patent Publication No. 9963/73.
However, such methods have been found to have problems as progress has been made on reducing the thickness of coated resin layer. That is, since the main purpose of coating resin layers on both surfaces of a paper support for waterproofing photographic paper is to prevent the permeation of a processing solution or water into the paper of the support, there is no specific lower limit on the thickness of the coated layers, but from an economic viewpoint, the thickness of the coated resin layers should be as thin as possible without reducing the water resisting property. Thus, investigations and efforts have been directed to thickness reduction of the coated resin layers and the industrial practice of using thin resin layers has advanced. However, with the progress of thickness reduction of coated resin layers, it has become difficult to control curling by utilizing the shrinking property of polyethylene as described above, and thus it has become difficult to obtain a support for waterproof photographic paper having excellent curling property by the above-described methods when using layers of reduced thickness.
On the other hand, Japanese Patent Application (OPI) No. 130626/77 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") provides a method for producing a support for waterproof photographic paper having an improved curling property by drying paper at different drying speeds at each surface when making the paper, to make a paper having curling, and then by forming resin layers on the surfaces by coating.
However, by the foregoing method, the reproducibility, i.e., obtaining a certain extent of curling of a paper, is difficult to obtain: or, in other words, even if the conditions for drying both surfaces of the support are sufficiently controlled, the extent of curling of the paper may deviate, and hence the efficiency of the method is unsatisfactory from a practical viewpoint.
It has also known to impart water resisting property to a paper support by coating an organic compound having an unsaturated bond capable of being polymerized by the irradiation of electron beams on the paper support in place of polyolefin and hardening the coated organic compound by the irradiation of electron beams as disclosed in Japanese Patent Application (OPI) Nos. 27257/82 (corresponding to U.S. Pat. No. 4,384,040, hereinafter the same), 30830/82 (U.S. Pat. No. 4,364,971) and 49946/82 (U.S. patent application Ser. No. 300,526). However, a method of controlling curling by using an electron beam-hardenable unsaturated organic compound has not been known.